Insitu epitaxial deposition of front and back junctions in single crystal silicon solar cells

ABSTRACT

Fabrication of a single crystal silicon solar cell with an insitu epitaxially deposited very highly doped p-type silicon back surface field obviates the need for the conventional aluminum screen printing step, thus enabling a thinner silicon solar cell because of no aluminum induced bow in the cell. Furthermore, fabrication of a single crystal silicon solar cell with insitu epitaxial p-n junction formation and very highly doped n-type silicon front surface field completely avoids the conventional dopant diffusion step and one screen printing step, thus enabling a cheaper manufacturing process.

This application claims the benefit of U.S. Provisional Application Ser.No. 61/454,363 filed Mar. 18, 2011, incorporated by reference in itsentirety herein.

FIELD OF THE INVENTION

The present invention relates generally to solar cells, and moreparticularly to methods for epitaxially depositing single crystalsilicon solar cells including epitaxially deposited front and backjunctions.

BACKGROUND

There is a lower limit to the thickness of single crystal silicon solarcells manufactured with an aluminum back surface field (BSF), since theAl BSF fabrication process, which involves screen printing an Al paste,induces a bow in thin silicon wafers when the Al paste is fired. For 200micron thick wafers bow starts to affect solar cell yield, and forwafers of 150 microns and less wafer bow becomes a yield killer forsolar cell fabrication. The Al paste shrinks during firing causing thewafer to bow such that the Al covered surface becomes convex. This waferbow may result in wafer breakage during subsequent processing,particularly during tabbing and stringing, and this is becoming agreater concern, as the solar industry migrates to larger wafers, from125 mm to 156 mm square (or pseudosquare) wafers, for example. There isa need for a manufacturable alternative to an Al BSF for making thinnersingle crystal silicon solar cells.

The front-side p-n junction in single crystal silicon solar cells iscurrently manufactured using a diffusion process, which also requires apost-diffusion clean. There is a need for a more efficient manufacturingprocess which avoids front side diffusion and clean.

SUMMARY OF THE INVENTION

A single crystal silicon solar cell with an insitu epitaxially depositedp⁺⁺ silicon BSF (for a p-base cell) will obviate the need for theconventional Al screen printing step, thus enabling a thinner siliconsolar cell because of no Al induced bow in the cell. Here the term p⁺⁺is used to refer to very highly p-doped silicon where the dopantconcentration is greater than 1×10¹⁸ cm⁻³ and the resistivity is lessthan or equal to 20 mohm-cm. This invention is applicable to both n- andp-base silicon solar cells.

Furthermore, a single crystal silicon solar cell with insitu epitaxialp-n junction formation and n⁺⁺ front surface field (FSF) completelyavoids the conventional dopant diffusion step and one screen printingstep, thus enabling a cheaper manufacturing process. Here the term n⁺⁺is used to refer to very highly n-doped silicon—with dopantconcentration of greater than 1×10¹⁸ cm⁻³, where the resistivity may beless than or equal to 20 mohm-cm. This invention is applicable to bothn- and p-base silicon solar cells.

According to aspects of the invention, a method of fabricating a thinepitaxial silicon solar cell may comprise: depositing an epitaxial filmof highly doped p-type silicon on a porous silicon layer on a siliconwafer, the highly doped p-type silicon film having a resistivity of lessthan 20 mohm-cm, the highly doped p-type silicon film being a backsurface field (BSF) layer; depositing an epitaxial film of p-typesilicon on the BSF, the p-type silicon film being a base layer;exfoliating the BSF and the base from the silicon wafer; forming anemitter layer at the surface of the base layer; forming front contactsto the emitter layer on the front surface of the cell; and forming backcontacts to the BSF on the back surface of the cell, the back contactsbeing patterned to cover less than fifty percent of the back surface ofthe cell. Furthermore, the front and back contact grids may be made ofthe same metal, may have the same dimensions and/or may be alignedfront-to-back.

According to further aspects of the invention, a method of fabricating athin epitaxial silicon solar cell may comprise: depositing an epitaxialfilm of highly doped p-type silicon on a porous silicon layer on asilicon wafer, the highly doped p-type silicon film having a resistivityof less than 20 mohm-cm, the highly doped p-type silicon film being anemitter layer; depositing an epitaxial film of n-type silicon on theemitter layer, the n-type silicon film being a base layer; depositing anepitaxial film of highly doped n-type silicon on the base layer, thehighly doped n-type silicon film having a dopant density of greater than1×10¹⁸ cm⁻³, the highly doped n-type silicon film being a front surfacefield (FSF) layer; exfoliating the emitter, the base and the FSF fromthe silicon wafer; forming front contacts to the FSF layer on the frontsurface of the cell; and forming back contacts to the emitter on theback surface of the cell, the back contacts being patterned to coverless than fifty percent of the back surface of the cell. Furthermore,the front and back contact grids may be made of the same metal, may havethe same dimensions and/or may be aligned front-to-back.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the present invention willbecome apparent to those ordinarily skilled in the art upon review ofthe following description of specific embodiments of the invention inconjunction with the accompanying figures, wherein:

FIG. 1 is a cross-sectional view of a representation of a conventionalsilicon solar cell;

FIG. 2 is a cross-sectional view of a representation of an epitaxialsolar cell, according to some embodiments of the present invention; and

FIG. 3 is a cross-sectional view of a representation of a furtherembodiment of an epitaxial silicon solar cell according to the presentinvention.

DETAILED DESCRIPTION

Embodiments of the present invention will now be described in detailwith reference to the drawings, which are provided as illustrativeexamples of the invention so as to enable those skilled in the art topractice the invention. Notably, the figures and examples below are notmeant to limit the scope of the present invention to a singleembodiment, but other embodiments are possible by way of interchange ofsome or all of the described or illustrated elements. Moreover, wherecertain elements of the present invention can be partially or fullyimplemented using known components, only those portions of such knowncomponents that are necessary for an understanding of the presentinvention will be described, and detailed descriptions of other portionsof such known components will be omitted so as not to obscure theinvention. In the present specification, an embodiment showing asingular component should not be considered limiting; rather, theinvention is intended to encompass other embodiments including aplurality of the same component, and vice-versa, unless explicitlystated otherwise herein. Moreover, applicants do not intend for any termin the specification or claims to be ascribed an uncommon or specialmeaning unless explicitly set forth as such. Further, the presentinvention encompasses present and future known equivalents to the knowncomponents referred to herein by way of illustration.

FIG. 1 shows a conventional solar cell with a screen-printed aluminumback contact prior art silicon solar cell. The cell of FIG. 1 comprisesan aluminum BSF and back contact 110, a p-type base 120, a diffusiondoped n⁺ emitter 130, an anti-reflection coating 140 and a silver frontcontact grid 150. The front surface was texture etched prior to formingthe emitter.

FIG. 2 shows a thin epitaxial solar cell with a p⁺⁺ in-situ backcontact, according to the present invention. FIG. 2 shows a siliconsolar cell comprising a back contact grid 210, a p⁺⁺ BSF 212, a p-typebase 220, a diffusion doped n⁺ emitter 230, an anti-reflection coating240 and silver front contact grid 250. The front surface was textureetched prior to forming the emitter. For ease of comparison, FIG. 2 isshown below FIG. 1 which shows a conventional solar cell with ascreen-printed aluminum back contact. Furthermore, due to the highconductivity of the p⁺⁺ layer, the back contacts of FIG. 2 may bepatterned to cover less than fifty percent of the back surface of saidcell, and preferably less than ten percent. The back contacts may beformed as a grid, for example, rather than a continuous layer, thelatter being required in the conventional cell of FIG. 1. Yetfurthermore, the contacts on the front and back surfaces of FIG. 2 maybe formed as matching grids and may also be formed of the samematerial—Ag, for example—resulting in little if any bow in the wafer.Matching grids may be grids that have the same line widths, heights andspacings and the same surface coverage; furthermore, matching grids mayalso be aligned front to back as shown in FIG. 2.

An embodiment of a process flow according to the present invention for aless than 200 micron thick bifacial solar cell, such as shown in FIG. 2includes the following steps:

-   -   1. form a porous silicon layer on a silicon substrate by anodic        etching in an HF-based solution;    -   2. anneal the porous silicon layer in H₂ gas in an epitaxial        deposition reactor;    -   3. deposit an epitaxial film of p⁺⁺ silicon BSF (resistivity of        1-20 mohm-cm, and preferably 1-10 mohm-cm) on the annealed        surface of the porous silicon, approximately 1-10 microns thick,        in the epitaxial deposition reactor;    -   4. deposit an epitaxial film of p-type silicon (0.5-2 ohm-cm        resistivity) on the BSF, approximately 40-200 microns thick, in        the epitaxial deposition reactor;    -   5. exfoliate the epitaxial silicon cell structure from the        silicon substrate and reclaim and reuse the silicon substrate        (this works for cells as thin as 80-90 microns which can be        processed free standing; thinner cells require support such as a        handle and/or may continue some of the front side processing        prior to exfoliation—see, for example U.S. Provisional Patent        Appl. No. 61/514,641, incorporated by reference herein);    -   6. further processing steps for the exfoliated silicon cell        structure include:        -   a. texture etch the front side, that is the surface of the            p-type silicon layer, using well known processes, using            solutions containing potassium hydroxide (KOH) and isopropyl            alcohol (IPA), for example;        -   b. diffuse an n-type dopant into the texture etched surface            to form a p-n junction;        -   c. deposit a 70-90 nm thick SiN_(X) film on the doped            textured surface using a plasma-enhanced chemical vapor            deposition (PECVD) or by reactive sputtering—the silicon            nitride layer acts as an anti-reflection coating (ARC) and            preferably has a refractive index close to 2 to give good            anti-reflection performance;        -   d. form on the front side a Ag grid with a busbar using            screen printing of Ag paste followed by drying the paste            (front side grids are formed so as to cover the minimum of            the front surface of the solar cell and yet provide an            effective electrical contact to the emitter); and        -   e. form on the back side a Ag or Ag/Al grid with a busbar,            using screen printing of metal paste followed by firing at            800 to 1,000 degrees C.—the front and back metallizations            are co-fired.

Crystal Solar's epitaxial reactor, as described in U.S. PatentApplication Publications Nos. 2010/0215872 and 2010/0263587, bothincorporated by reference herein, provides a low cost, high throughputmeans for epitaxial silicon deposition which can be utilized for theabove epitaxial deposition steps. The above process may also readily beadapted to make an n-base cell. Furthermore, variations on the aboveprocess flow may include alternative materials and deposition methodsfor the front side and back side electrical contacts. The porous siliconlayer may have modulated porosity, with a lower porosity at the surface.Further variations are discussed in U.S. Patent Application PublicationNo. 2012/0040487 and U.S. patent application Ser. No. 13/241,112, bothincorporated by reference herein. Yet further variations will beapparent to those skilled in the art after reading the disclosure of thepresent invention.

The epitaxial solar cell design of the present invention, as shown inFIG. 2, is important since it completely avoids the Al screen printingstep, instead using a p⁺⁺ layer in the back of the cell to allow ohmiccontact to the Ag or Ag/Al grid. The epitaxial cell of the presentinvention may include the following advantages over a conventional cell:lower cell manufacturing cost since Al screen printing is avoided;thinner silicon (below 200 microns, and particularly below 150 microns)is enabled because Al back contact induced bow is avoided; the epitaxialcell of the present invention can be used as a bifacial cell with doubleglass, such as described in U.S. Patent Application Publication No.2011/0056532 and U.S. Provisional Patent Application No. 61/514,641,both incorporated by reference herein; and the performance of a cellwith an epitaxial silicon BSF is expected to be improved over a cellwith an Al screen printed BSF—the former being expected to have a higheropen circuit voltage, V_(oc).

FIG. 3 shows a schematic representation of a thin single crystal siliconsolar cell with a p++ silicon emitter and an epitaxial n⁺⁺ FSF layer,according to the present invention. FIG. 3 shows a silicon solar cellcomprising a back contact grid 310, a p⁺⁺ emitter 312, an n-type base320, an epitaxially deposited n⁺⁺ FSF 330, an anti-reflection coating340 and silver front contact grid 350. The front surface was textureetched after depositing the FSF. Furthermore, due to the highconductivity of the p⁺⁺ layer, the back contacts of FIG. 3 may bepatterned to cover less than fifty percent of the back surface of saidcell, and preferably less than ten percent. The back contacts may beformed as a grid, for example, rather than a continuous layer, thelatter being required in the conventional cell of FIG. 1. Yetfurthermore, the contacts on the front and back surfaces of FIG. 3 maybe formed as matching grids and may also be formed of the samematerial—Ag, for example—resulting in little if any bow in the wafer.Matching grids may be grids that have the same line widths, heights andspacings and the same surface coverage; furthermore, matching grids mayalso be aligned front to back as shown in FIG. 3.

An embodiment of a process flow according to the present invention for aless than 200 micron thin bifacial solar cell, such as shown in FIG. 3includes the following steps:

-   -   1. form a porous silicon layer on a silicon substrate by anodic        etching in an HF-based solution;    -   2. anneal the porous silicon layer in H₂ gas in an epitaxial        deposition reactor;    -   3. deposit an epitaxial film of p⁺⁺ silicon emitter (resistivity        of 1-20 mohm-cm, and preferably 1-10 mohm-cm) on the annealed        surface of the porous silicon, approximately 0.5 microns or less        in thickness, in the epitaxial deposition reactor;    -   4. deposit an epitaxial film of n-type silicon (0.5-2 ohm-cm        resistivity) on the emitter, approximately 50-200 microns thick,        in the epitaxial deposition reactor;    -   5. deposit an epitaxial film of n⁺⁺ silicon FSF (with dopant        density of greater than 1×10¹⁸ cm⁻³, which may provide a        resistivity in the range of 1-20 mohm-cm, and preferably 1-10        mohm-cm) on the n-type silicon layer, approximately 10-20        microns thick;    -   6. exfoliate the epitaxial silicon cell structure from the        silicon substrate and reclaim and reuse the silicon substrate        (this works for cells as thin as 80-90 microns which can be        processed free standing; thinner cells require support such as a        handle and/or may continue some of the front side processing        prior to exfoliation—see, for example U.S. Provisional Patent        Appl. No. 61/514,641, incorporated by reference herein);    -   7. further processing steps for the exfoliated silicon cell        structure include:        -   a. texture etch the front side, that is the surface of the            n⁺⁺ FSF (with p⁺⁺ junction protected), using well known            processes, using solutions containing potassium hydroxide            (KOH) and isopropyl alcohol (IPA), for example;        -   b. deposit a 70-90 nm thick SiN_(X) film on the textured            surface using a plasma-enhanced chemical vapor deposition            (PECVD) or by reactive sputtering—the silicon nitride layer            acts as an anti-reflection coating (ARC) and preferably has            a refractive index close to 2 to give good anti-reflection            performance;        -   c. form on the front side a Ag grid with a busbar using            screen printing of Ag paste followed by drying the paste            (front side grids are formed so as to cover the minimum of            the front surface of the solar cell and yet provide an            effective electrical contact to the FSF); and        -   d. form on the back side a Ag or Ag/Al grid with a busbar,            using screen printing of metal paste followed by firing at            800-1,000 degrees C.—the front and back metallizations are            co-fired.

Crystal Solar's epitaxial reactor, as described in U.S. PatentApplication Publications Nos. 2010/0215872 and 2010/0263587, bothincorporated by reference herein, provides a low cost, high throughputmeans for epitaxial silicon deposition which can be utilized for theabove epitaxial deposition steps.

The epitaxial solar cell design of the present invention, as shown inFIG. 3, is important since: complete front and back junction formationin epitaxial silicon completely avoids the diffusion process and postdiffusion cleans, hence has much lower cost; these cells are highefficiency n-type cells; and these cells may be made with less than 150microns thickness, due to having no Al back contact—see discussion abovewith respect to the cell structure of FIG. 2.

Further details of fabrication methods for epitaxial silicon solar cellsare provided in U.S. Pat. No. 8,030,119, US Patent ApplicationPublications Nos. 2010/0108134, 2010/0108130, 2011/0056532 and2012/0040487 and U.S. patent application Ser. No. 13/241,112 forexample, all of which are incorporated by reference.

Although methods of the present invention have been described with themonocrystalline silicon wafer of thickness less than 200 microns(preferably 100-140 microns) being formed by epitaxial deposition on aporous silicon layer on the surface of a silicon substrate followed byexfoliation, where the porous silicon layer acts as a fracture layer,other methods of forming the monocrystalline silicon wafer may be used.For example, the less than 200 micron silicon substrates may be formedby exfoliation from a silicon substrate where proton implantation to adesired depth followed by annealing at a suitable temperature can beused to exfoliate the thin silicon substrate. Furthermore, thin siliconsubstrates may be formed by mechanical sawing and or polishing of asilicon substrate or boule.

Although methods of the present invention have been described with thep⁺⁺ BSF being formed by epitaxial deposition, other methods for formingthe BSF may be used. For example, the BSF may be formed by ionimplantation of boron or by diffusion of boron into the back surface ofthe wafer (such as by exposing the back side of the wafer to BBr₃ orBCl₃ at a high temperature in a diffusion furnace).

Although methods of the present invention have been described with frontand back contact grids formed by depositing metal paste and firing, thefront and/or back contact grids may also be formed by other techniquesincluding electroplating of metals and alloys, such as copper (using asuitable barrier metallurgy such as Ni followed by copper plate-up).

Furthermore, these alternative fabrication methods may be combinedtogether to form solar cells such as those of FIGS. 2 & 3. For example,a thin silicon substrate formed by mechanical sawing may have a BSFformed on the back surface by ion implantation of boron, front sideprocessing as per the description for FIG. 2 given above, and the frontand back metal contact grids may be formed by electroplating of copperto fabricate the solar cell of FIG. 2.

The solar cells described herein are silicon-based solar cells, and theteaching and principles of the present invention apply to solar cellscomprising single crystal silicon, multicrystalline silicon, and siliconheterojunctions.

Although the present invention has been particularly described withreference to certain embodiments thereof, it should be readily apparentto those of ordinary skill in the art that changes and modifications inthe form and details may be made without departing from the spirit andscope of the invention.

What is claimed is:
 1. A method of fabricating a thin epitaxial silicon solar cell comprising: depositing by chemical vapor deposition an epitaxial film of highly doped p-type silicon on a porous silicon layer on a silicon wafer, said highly doped p-type silicon epitaxial film having a resistivity of less than 20 mohm-cm, said highly doped p-type silicon epitaxial film being a back surface field (BSF) layer; depositing by chemical vapor deposition an epitaxial film of p-type silicon on said BSF layer, said p-type silicon epitaxial film being a base layer; exfoliating said BSF layer and said base layer from said silicon wafer; forming an emitter layer at the surface of said base layer; forming front contacts to said emitter layer on the front surface of said thin epitaxial silicon solar cell; and forming back contacts to said BSF layer on the back surface of said thin epitaxial silicon solar cell, said back contacts being patterned to cover less than fifty percent of the back surface of said thin epitaxial silicon solar cell; wherein said thin epitaxial silicon solar cell is less than 200 microns thick.
 2. The method of claim 1, wherein said thin epitaxial silicon solar cell is less than 150 microns thick.
 3. The method of claim 1, wherein said back contacts cover less than ten percent of the back surface of said thin epitaxial silicon solar cell.
 4. The method of claim 1, wherein said front contacts and said back contacts are formed of the same metal.
 5. The method of claim 4, wherein said metal comprises silver.
 6. The method of claim 1, wherein said front contacts and said back contacts are patterned as grids.
 7. The method of claim 6, wherein the front contact grid and the back contact grid have the same dimensions.
 8. The method of claim 7, wherein the front contact grid and the back contact grid are aligned front-to-back.
 9. The method of claim 1, further comprising texture etching the surface of said base layer before said forming said emitter layer.
 10. The method of claim 1, wherein said highly doped p-type silicon epitaxial film has a resistivity of less than 10 mohm-cm.
 11. The method of claim 1, wherein said emitter layer is formed by diffusing an n-type dopant into said base layer.
 12. The method of claim 1, wherein said thin epitaxial silicon solar cell is bifacial.
 13. A method of fabricating a thin epitaxial silicon solar cell comprising: depositing by chemical vapor deposition an epitaxial film of highly doped p-type silicon on a porous silicon layer on a silicon wafer, said highly doped p-type silicon epitaxial film having a resistivity of less than 20 mohm-cm, said highly doped p-type silicon epitaxial film being an emitter layer; depositing by chemical vapor deposition an epitaxial film of n-type silicon on said emitter layer, said n-type silicon epitaxial film being a base layer; depositing by chemical vapor deposition an epitaxial film of highly doped n-type silicon on said base layer, said highly doped n-type silicon epitaxial film having a dopant density of greater than 1×10¹⁸ cm⁻³, said highly doped n-type silicon epitaxial film being a front surface field (FSF) layer; exfoliating said emitter layer, said base layer and said FSF layer from said silicon wafer; forming front contacts to said FSF layer on the front surface of said thin epitaxial silicon solar cell; and forming back contacts to said emitter layer on the back surface of said thin epitaxial silicon solar cell, said back contacts being patterned to cover less than fifty percent of the back surface of said thin epitaxial silicon solar cell; wherein said thin epitaxial silicon solar cell is less than 200 microns thick.
 14. The method of claim 13, wherein said thin epitaxial silicon solar cell is less than 150 microns thick.
 15. The method of claim 13, wherein said back contacts cover less than ten percent of the back surface of said thin epitaxial silicon solar cell.
 16. The method of claim 13, wherein said front contacts and said back contacts are formed of the same metal.
 17. The method of claim 16, wherein said metal comprises silver.
 18. The method of claim 13, wherein said front contacts and said back contacts are patterned as grids.
 19. The method of claim 18, wherein the front contact grid and the back contact grid have the same dimensions.
 20. The method of claim 19, wherein the front contact grid and the back contact grid are aligned front-to-back.
 21. The method of claim 13, further comprising texture etching the surface of said FSF layer and depositing an anti-reflection coating on said FSF layer before said forming said front contacts.
 22. The method of claim 13, wherein said highly doped p-type silicon epitaxial film has a resistivity of less than 10 mohm-cm.
 23. The method of claim 13, wherein said highly doped n-type silicon epitaxial film has a resistivity of less than 10 mohm-cm.
 24. The method of claim 13, wherein said thin epitaxial silicon solar cell is bifacial. 